A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting. Accordingly, there is an ongoing need to provide lighting which is more energy-efficient.
It is well known, however, that any proposed new (or existing) lighting device must adequately deal with heat generated by the light source employed in the lighting device. The present inventive subject matter provides heat transfer structures and heat transfer elements which assist in addressing heat generation issues in lighting devices, and lighting devices which include such heat transfer structures and heat transfer elements.
Light sources which are showing great promise are solid state light emitters, e.g., light emitting diodes. It is well known that incandescent light bulbs are very energy-inefficient light emitters—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction.
Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs.
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, efficacy (lm/W), and/or duration of service.
The need to adequately remove heat generated by the light source is particularly pronounced with respect to solid state light emitters. LED light sources, for example, have operating lifetimes of decades (as opposed to just months or one or two years for many incandescent bulbs), but an LED's lifetime is usually significantly shortened if it operates at elevated temperatures. It is generally accepted that the junction temperature of an LED should not exceed 85 degrees C. if a long lifetime is desired.
In addition, the intensity of light emitted from some solid state light emitters varies based on ambient temperature. For example, LEDs which emit red light often have a very strong temperature dependence (e.g., AlInGaP LEDs can reduce in optical output by ˜20% when heated up by ˜40 degrees C., that is, approximately −0.5% per degree C.; and Blue InGaN+YAG:Ce LEDs can reduce by about −0.15%/degree C.).
As is well known, in many instances where lighting devices include solid state light emitters as light sources (e.g., general illumination devices which emit white light in which the light sources consist of light emitting diodes), a plurality of solid state light emitters are provided which emit light of different colors which, when mixed, are perceived as the desired color for the output light (e.g., white or near-white). As noted above, the intensity of light emitted by many solid state light emitters, when supplied with a given current, can vary as a result of temperature change. The desire to maintain a relatively stable color of light output is therefore an important reason to try to reduce temperature variation of solid state light emitters.
In addition, the potential for variation in intensity of solid state light emitters (e.g., depending on the ambient temperature and/or the age of the solid state light emitter) has in many instances led to the inclusion in some lighting devices which include solid state light emitter of one or more sensors which detect (1) the color of the light being emitted from the lighting device, and/or (2) the intensity of the light being emitted from one or more of the solid state light emitters, and/or (3) the intensity of light of one or more specific hues of color. By providing such sensors, it is possible to adjust the current supplied to one or more of the solid state light emitters, based on the readings from such sensor(s), in order to maintain the color of the output light within a desired range of color.